Comment: I've been teaching some basic piano to my girlfriend this evening, and
while looking for references I've noticed that the Intertubes abound with
a persistent rumor: that the tritone, aka the Devil's Interval (C-F# for
example, or the first two chords in Jimi Hendrix' "Purple Haze") has been
banned by the Catholic Church, from the Middle Ages onward. For what it's
worth, Wikipedia pooh-poohs the story, but there are a zillion people who
perpetuate it without question. At the very least I would love to find out
whence came this urban legend (if it is indeed an urban legend).

The tritone was in fact referred to as "diabolus in musica," or "the Devil in music," for its extreme dissonance. Guido D'Arezzo (the monk whose "Guidonian Hand" system of solfege singing and modulation now lives on in posters adorning the walls of pretentious music geeks everywhere) expressly excluded the interval from his system. It was (and still often is) discouraged in melodic lines because it's a somewhat awkward interval to sing, and since Gregorian chant is nothing but unison melodic lines, it was in fact more or less effectively "banned by the Catholic church" during the earliest centuries of liturgical music. (The church was pretty much the outlet for composed vocal music during that time.)

Harmonically, the interval is quite useful (it's what gives the "pull" to dominant seventh chords), and Jazz players make extensive use of chord substitutions based on the interval. The very melodic awkwardness that kept it out of chant makes it appealing in many instances of more modern music. It's the interval between the first two notes of "Maria" from West Side Story (also "Cool"), as well as the first two notes of the vocal opening to the theme from The Simpsons. It features prominently in Alan Silvestri's theme from Back to the Future (The second to third note of the theme, following a descending fifth, in case anyone was wondering).

However, tales of medieval monks being excommunicated, whipped, or burned at the stake are probably exaggerated. My best guess as to why these legends persist is that it's a great way for music history and music theory professors to keep their undergraduate students' attention and drive home the idea that writing melodic tritones in that style is considered bad practice.

Start with the well known Cmaj scale:
C D E F G A B C, then delete the F and B:and you're left with C D E G A C.
You can even choose a different starting point: A C D E G A
This is called the Pentatonic scale (Major or minor is really just a different starting point for the same notes.

Play with just these 5 notes, you have songs like Amazing Grace and the theme song to Gilligan's Island, and can keep your soul intact, there's no tritonus to endanger it.